The present invention relates to a semiconductor device, and more particularly to a semiconductor having improved characteristics.
It is known that glass substrate of a solar cells having textured surfaces improve the light absorption characteristics of the solar cell by scattering the light. It is also known that if the thickness of an SnO.sub.2 film is increased which is used as the transparent conductive film (transparent electrode) then the dimension of the reliefs between peaks and troughs thereof increases.
Photoelectric conversion efficiency (hereinafter referred to as FF), however, is greatly reduced when the relief between peaks and troughs of glass substrates is too large since the ratio of the internal series resistance of a solar cell becomes small. The relief dimension of a glass substrate is usually represented by the Haze ratio which is defined by the following formula: ##EQU1## When the thickness of SnO.sub.2 film itself is increased light absorption of SnO.sub.2 becomes large and the enhancement of the light absorption due to the textured surface is negated, so that efficiency of the solar cell is reduced. As a result, the transparent conductive film of a conventional solar cell usually has a thickness of between 5000 .ANG. and 7000 .ANG. and dopant density is between 1 and 2 weight %.
The light absorption coefficient of the transparent conductive film can be lowered by reducing the content of dopant, and the properties of the solar cell are improved without increasing light absorption of the transparent electrode even if a thick transparent conductive film is used in the solar cell.
However, the FF of a solar cell having low dopant transparent conductive film is not improved because increased contact resistance between the transparent electrode and the first layer of the amorphous semiconductor layers occurs. The first layer of the amorphous semiconductor layer is for example, p-type layer an i-type and an n-type semiconductor layer formed on the transparent electrode. The output current of multi-junction solar cell having a number of amorphous semiconductor layers are stacked to obtain high output current. Output current is defined by 1/number of blocks and output loss is defined by (1/number of blocks).sup.2. So that a transparent electrode having higher resistance can be employed. The resistivity of transparent electrode is normally controlled by thickness thereof, i.e., the resistivity increases in accordance with a decrease in thickness.
As a result, it is understandable that thinner transparent electrodes are employed for multi-junction solar cells as compared with single-junction solar cells. It is expected that output power will increase if thinner transparent electrode, are employed since light absorption by transparent electrodes is decreased.
However, the output power of a solar cell employing a thinner transparent electrode is not increased as is expected, because, as is mentioned above, the light scattering effect of the textured surface of transparent electrode is reduced in accordance with reduction in the transparent electrode thickness.
Taking the above mentioned matters into consideration, it is an object of the present invention to provide a semiconductor device of in which light absorption of transparent electrodes is improved.
Further, it is an object of the present invention to provide a semiconductor device wherein the contact resistance between the transparent electrode and the amorphous semiconductor layers is lowered.
Still further, it is an object of the present invention to provide a multi-junction solar cell in which output power is enhanced.